linux-capture: Clear x events when exiting handler

This is probably not necessary but might fix an issue where errors pass
through to other parts of the program, possibly causing the crash on
exit related to the xcomposite capture.

plugins/linux-capture/xcompcap-helper.cpp

@@ -373,6 +373,8 @@ void XErrorLock::lock()
 void XErrorLock::unlock()
 {
 	if (islock) {
+		XSync(XCompcap::disp(), 0);
+
 		curErrorTarget = 0;
 		XSetErrorHandler(prevhandler);
 		prevhandler = 0;

plugins/linux-capture/xcompcap-main.cpp

@@ -234,6 +234,9 @@ static Window getWindowFromString(std::string wstr)
 
 static void xcc_cleanup(XCompcapMain_private *p)
 {
+	PLock lock(&p->lock);
+	XErrorLock xlock;
+
 	if (p->gltex) {
 		gs_texture_destroy(p->gltex);
 		p->gltex = 0;